1. Field of the Invention
The present invention relates to an optical transmission medium, and more particularly to an optical fiber.
2. Description of the Related Art
Conventional processes for manufacturing optical fibers are generally divided into a preform manufacturing process and a drawing process. In the drawing process, the drawing temperature that is used has a great influence on residual stress, which occurs at an interface between a core and a clad of the optical fibers. Varying residual stress affects various optical characteristics. Therefore, it is of a great importance to control the residual stress using the drawing temperature. Otherwise, composition of the optical fibers should be designed in such a manner that the residual stress is not affected by the drawing temperature.
However, optical fibers (including Ethernet optical fibers of 10 Mbps, 100 Mbps, 1 Gbps, and 10 Gbps), which are mainly used as optical transmission media for an access network, a FTTH (fiber-to-the-home), a LAN, or a WAN, are supposed to pass through many curved areas when they are installed in conduit lines, buildings, or complicates office rooms for installation and operation. Such curved areas should not affect the loss, which is one of the important factors in communications.
FIG. 1 shows a typical optical fiber having a graded index distribution. The optical fiber 110 includes a core 120 and a clad 130, which surrounds the core 120. The index distribution of the core 120 is given below as Equation 1 and the modal bandwidth of the optical fiber 110, which is one of the important optical characteristics, is greatly influenced by the value of α in the equation.n=n1(1−2Δ(r/α)α)1/2  (Equation 1)
In the above equation, n is the index of the core 120 of the optical fiber 110; n1 is the index at the center (r=0) of the core 120; Δ is a relative index difference; a is the radius of the core 120; r is a radius measured from the center of the core 120; and α is a core form factor. The α is sensitive to the residual stress discontinuity at the interface (r/a=1) between the core 120 and the clad 130. In particular, the residual stress discontinuity is greatly influenced by a difference in composition between the core 120 and the clad 130 or by the optical fiber's drawing conditions, such as heating temperature, cooling rate, and drawing rate.
Conventional technologies regarding bending loss of optical fibers include methods for reducing the bending loss of single-mode optical fibers, mainly by modifying the index structure of their core and clad. However, no method is known in the art for controlling the residual stress discontinuity.
U.S. Pat. No. 4,412,722 of Carnevale, et al, entitled “Single-mode fiber with graded index of refraction,” discloses a method for fabricating a single-mode optical fiber having a core with a graded index distribution. U.S. Pat. No. 4,838,643 of Hodges, et al, entitled “Single-mode bend insensitive fiber for use in fiber optic guidance applications,” discloses a structure having depressed regions or isolated trenches in parts of a clad area. U.S. Pat No. 5,032,001 of Shang, entitled “Optical fiber having enhanced bend resistance,” discloses a structure having raised areas and depressed areas in a clad area. U.S. Pat. No. 5,278,931 of Antos, et al, entitled “Low bend loss single-mode optical waveguide fiber,” discloses a method wherein the index of an inner core is increased to reduce a MFD (mode field diameter), while maintaining zero-dispersion wavelength; a diffusion tail effect is reduced at an interface between a core and a clad to suppress the increase in an unnecessary cut-off wavelength and the MFD; or a ring structure is applied to an external area to suppress the increase in the zero-dispersion wavelength, which is caused by the increase in an outer core index. U.S. Pat. No. 5,175,785 of Dabby, entitled “Optical waveguides having reduced bending loss and method of making the same,” discloses a structure capable of supporting a dual mode or a multi mode by using a virtual single-mode structure, the cut-off wavelength of which is substantially larger than the zero-dispersion wavelength, and decreasing the index difference between a core and a clad thereof.
As mentioned above, conventional optical fibers have a problem in that they exhibit severe changes in residual stress, depending on drawing conditions, at an interface between their core and clad. In addition, conventional optical fibers have large bending loss even in the case of a small bending (diameter of about 10 mm), which occurs at recent access networks, FTTHs, LANs, or WANs.